Radio systems provide users of radio subscriber units with wireless communications. A particular type of radio system is a cellular radiotelephone system. A particular type of radio subscriber unit is a cellular radiotelephone subscriber unit, sometimes referred to as a mobile station. Cellular radiotelephone systems generally include a switch controller coupled to a public switched telephone network (PSTN) and a plurality of base stations. Each of the plurality of base stations generally defines a geographic region proximate to the base station to produce coverage areas. One or more mobile stations communicate with a base station that facilitates a call between the mobile station and the public switched telephone network. A description of a cellular radiotelephone system is described in the book "Mobile Cellular Communications Systems" by Dr. William C. Y. Lee, 1989.
Some mobile stations have diversity to improve the reception of communication signals sent from the base station. Diversity employs equipment redundancy or duplication to achieve an improvement in receiver performance under multipath fading conditions. Space diversity, in particular, employs two or more antennas that are physically spaced apart by a distance related to the wavelength. In a space diversity system, a transmitted signal travels by slightly different paths from the transmitter to the two antennas at the receiver. In addition, there may be reflected paths, where the transmitted signal received by each antenna has also traveled by different paths from the transmitter. Experience has shown that when the reflected path causes fading by interference with the transmitted signal, the two received signals may not be simultaneously affected to the same extent by the presence of multipath fading, because of the different paths. Although the path from the transmitter to one of the two antennas may cause phase cancellation of the transmitted and reflected path waves, it is less probable that multiple paths to the other antenna will cause phase cancellation at the same time. The probability that the two antennas are receiving exactly the same signal is called a correlation factor.
Known space diversity systems include switched antenna diversity (SAD), selection diversity (SD) and maximal ratio combining diversity (MRCD). Each diversity system includes a controller having an algorithm programmed therein for controlling the diversity system. A detailed comparison of these three diversity systems is described in "On The Optimization Of Simple Switched Diversity Receiver" by Zdunek et al, 1978 IEEE Canadian Conference on Communications and Power, Montreal, Canada and "Performance And Optimization Of Switched Diversity Receivers" by Zdunek et al, IEEE Transactions on Communications, December 1979. A brief description of these three diversity systems is now provided.
SAD employs two antennas coupled to a single receiver through a single pole, double throw radio frequency (RF) switch. A controller samples the signal received from each antenna to couple only one of the two antennas to the receiver at a time.
SD employs two antennas and two receivers, wherein each antenna is coupled to its own receiver. The receiver with the highest baseband signal to noise ratio (SNR) is selected to be the demodulated signal. SD provides improved performance over SAD because the signals produced by the receivers can be monitored more often than with SAD and suffer fewer switching transients. However, a weakness of both SAD and SD is that only one antenna is used at any instant in time, while the other is disregarded.
MRCD also employs two antennas and two receivers, wherein each antenna is coupled to its own receiver. MRCD seeks to exploit the signals from each antenna by weighting each signal in proportion to their SNRs and then summing them. Accordingly, the individual signals in each diversity branch are cophased and combined, exploiting all the received signals, even those with poor SNRs. However a disadvantage of MRCD is that MRCD is more difficult and complicated to implement than SAD or SD.
A particular type of cellular radiotelephone system employs spread spectrum signaling. Spread spectrum can be broadly defined as a mechanism by which the bandwidth occupied by a transmitted signal is much greater than the bandwidth required by a baseband information signal. Two categories of spread spectrum communications are direct sequence spread spectrum (DSSS) and frequency-hopping spread spectrum (FHSS). The essence of the two techniques is to spread the transmitted power of each user over such a wide bandwidth (1-50 Mhz) that the power per unit bandwidth, in watts per hertz, is very small.
Frequency-hopping systems achieve their processing gain by avoiding interference, whereas the direct sequence systems use an interference attenuation technique. For DSSS, the objective of the receiver is to pick out the transmitted signal from a wide received bandwidth in which the signal is below the background noise level. The receiver must know the carrier frequency signal, type of modulation, pseudorandom noise code rate, and phase of the code in order to do this, since signal to noise ratios are typically minus 15 to 30 dB. Determining the phase of the code is the most difficult. The receiver uses a process known as synchronization to determine the starting point of the code from the received signal in order to despread the required signal while spreading all unwanted signals.
The DSSS technique acquires superior noise performance, compared to frequency hopping, at the expense of increased system complexity. The spectrum of a signal can be most easily spread by multiplying it with a wideband pseudorandom code-generated signal. It is essential that the spreading signal be precisely known so that the receiver can demodulate (i.e. despread) the signal. Furthermore, it must lock onto and track the correct phase of the received signal within one chip time (i.e. a partial or subinteger bit period). At the receiving end, a serial search circuit is used. There are two feedback loops, one for locking onto the correct code phase and the other for tracking the carrier. For code phase locking, the code clock and carrier frequency generator in the receiver are adjusted so that the locally generated code moves back and forth in time relative to the incoming received code. At the point which produces a maximum at the correlator output, the two signals are synchronized, meaning that the correct code phase has been acquired. The second loop (carrier tracking loop) then tracks the phase and frequency of the carrier to ensure phase lock is maintained.
A cellular radiotelephone system using DSSS is commonly known as a Direct Sequence Code Division Multiple Access (DS-CDMA) system. Individual users in the system use the same RF frequency but are separated by the use of individual spreading code.
In a DS-CDMA system a forward channel is defined as a communication path from the base station to the mobile station, and a reverse channel is defined as a communication path from the mobile station to the base station. The forward channel operation of DS-CDMA may be greatly improved by adding rake fingers to the receiver of the mobile station. The performance improvement provided by these extra rake fingers can approach the performance of MRCD by optimally exploiting resolvable delay spread and soft handoff. Unfortunately, field tests have measured only a small percentage of time where there is significant resolvable delay spread and both theory and simulations have shown soft handoff enhancement to be over a very limited amplitude range of the signal. As a result, the forward channel suffers performance degradation with respect to the reverse channel which has antenna diversity and takes full advantage of all its fingers.
Not only is there reduced range in the forward channel but the quality of the channel is poorer because frame error rate (FER) occurrences are correlated. Whereas reverse channel errors are much more random in time resulting in higher quality speech sound. The fundamental reason for the correlation is the character of the fading channel and the sluggishness of the forward channel power control loop.
Coherent antenna combining could solve the range imbalance issue and go a long way to eliminate the FER correlation problems. But coherent antenna combining is typically avoided in mobile stations because of the cost of receiver duplication and especially in DS-CDMA mobile stations because of the high complexity of the receiver.
SAD can be a solution. SAD is required in Personal Digital Cellular (PDC) mobile stations. But, their time division multiple access (TDMA) access method allows an antenna decision to be made just prior to a serving time slot arrival. No switching is permitted within the time slot. The Ardis.TM. Portable Data Terminal uses switched diversity that operates within the message; but, it is ineffective at speeds above 10 MPH. This is because the traditional switch algorithm can't keep up with the fast fades.
Accordingly, there is a need for a dual mode radio subscriber unit having a diversity receiver apparatus and method therefor that overcomes the disadvantages of the prior art and works well in DSSS systems.